Right-click on a message and (don't click, but) right-click "Share Message" and copy. You'll get a link that looks like:
https://matrix.to/#/!QtykxKocfZaZOUrTwp:matrix.org/!QtykxKocfZaZOUrTwp:matrix.org
The format of the link should be https://matrix.to/#/<room ID>/<event ID> but it is instead https://matrix.to/#/<room ID>/<room ID>.
Seems like a fairly high priority bug.
https://riot.im/develop appears to produce the correct URL with room and event as expected. Are there any other steps? What do you see if you try https://riot.im/develop?
Yep, same bug in riot.im/develop
Perhaps the particular message you are trying to share is relevant? Could you describe a room and message I can find through other means that exhibits this issue?
Any message is Matrix HQ is doing it for me.
Ah I think I see the issue. I'm not clicking Share Message, I'm right-clicking it in the menu and copying it. It's fine in the actual modal view that pops up but not in the message right-click menu.
Aha, yes, I agree, the link on the "Share Message" item itself is broken.